1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus. More particularly, the present invention relates to a developing device with the use of a two-component developer including a toner and a magnetic carrier, and an image forming apparatus such as an electrostatic copying machine, a laser printer and a facsimile machine that form images using the developing device by an electrophotographic method.
2. Description of the Related Art
In the image forming apparatus using the electrophotographic method, an electrostatic latent image is formed on a surface of a photoconductor drum (toner image holder), toner is supplied to the photoconductor drum by means of a developing device to develop the electrostatic latent image, a toner image formed on the photoconductor drum through the development is transferred onto a sheet such as a paper sheet, and the toner image is fused onto the sheet by means of a fuser.
In recent years, for a full-color compliant and high-definition compliant image forming apparatus, a two-component developer (hereinafter, may be referred to simply as “developer”) has been often used, which is excellent in toner charging stability. The developer includes a toner and a magnetic carrier. The toner and the carrier are agitated in a developing device to generate friction between the toner and the magnetic carrier, and the friction allows the toner to be appropriately charged.
The charged toner is supplied to a surface of a developer holder such as a developing roller. The toner supplied to the developing roller is transferred to an electrostatic latent image formed on a photoconductor drum by electrostatic attraction. Thus, a toner image based on the electrostatic latent image is formed on the photoconductor drum.
Further, such an image forming apparatus is required to be more high-speed and downsized. It is therefore necessary to quickly perform sufficient charging of a developer and to quickly convey the developer.
To this end, as a today's image forming apparatus, there has been proposed an image forming apparatus including a circulative developing device in order to instantly disperse supplementary toner into a developer to give an appropriate amount of charge. The circulative developing device includes a developer conveyance path being a route along which the developer is conveyed in a circulative manner, and a developer conveyance member provided in the developer conveyance path for agitating and conveying the developer at the same time (see Japanese Unexamined Patent Publication No. HEI 9 (1997)-120201).
In such a circulative developing device, the developer in the developer conveyance path is drawn by a developing roller, and the toner in the developer is supplied to the photoconductor drum. The developer in which the toner has been used for development separates from the surface of the developing roller to be returned into the developer conveyance path.
A problem of the developer in which the toner has been used for development is that it has a locally decreased toner concentration to locally decrease an image density when drawn again by the developing roller immediately after it is returned into the developer conveyance path.
In order to solve the problem, there has been proposed a developing device directed to reduction of an uneven image density due to imbalance of a developer, the developing device comprising: two developer conveyance paths communicated with each other; and a developing roller disposed above the conveyance paths, wherein the developer used for the development is not returned into the developer conveyance path for conveying the developer not yet used for the development but returned into the other developer conveyance path to reduce occurrence of a locally uneven toner concentration, and the developer conveyance path for conveying the developer not yet used for the development is provided with a conveyance member including a helical blade having helical pitches that are decreased toward the downstream side from the upstream side of a developer conveyance direction to stabilize the level of the surface of the developer in the developer conveyance path for conveying the developer not yet used for the development (see Japanese Unexamined Patent Publication No. 2010-197839).
In the developing device disclosed in Japanese Unexamined Patent Publication No. 2010-197839, however, the route of the developer circulation is so complicated that it is difficult to maintain the surface of the developer constant. In particular, when the charge amount of the toner is significantly increased due to influence of humidity or the like, the flowability of the developer is significantly reduced to raise the surface of the developer at the downstream side of the developer conveyance path. As a result, the amount of the developer to be drawn by the developing roller is increased, and therefore the amount of the developer to be supplied to the photoconductor drum is increased to increase the image density, causing an uneven image density in the axial direction of the developing roller.